This invention relates to the field of soil remediation, and in particular, to the evaporation of volatile contaminants through sub-surface soil heating.
An unfortunate byproduct of the rise of the chemical and petrochemical industries is that on occasion, toxic chemicals find their way into the soil. In the short-term, these toxic chemicals can devastate the local flora and fauna. Given enough time, these chemicals can percolate through the soil and into municipal water supplies where they can cause widespread illnesses in the local population. It is therefore important to quickly remove these chemicals from the contaminated soil.
A known method of removing volatile chemicals from contaminated soil is to heat the soil above the boiling point of those chemicals. This heating causes the chemicals to vaporize. The chemical vapors rising from the soil are then collected and condensed for proper disposal.
A known method of heating the soil is to bore pairs of holes into the soil and to insert electrodes into those holes. When a voltage is applied between the electrodes, current flows from one electrode, through the soil, and into the other electrode. As the current passes through the soil, it encounters resistance. This resistance results in the generation of heat.
A disadvantage of the foregoing soil heating method is that the soil is heated only along the current path between a pair of, electrodes. As a result, the temperature distribution in the soil is uneven. It is true that given enough time, heat will flow from hot portions of the soil to cooler portions of the soil, thereby equalizing the temperature distribution within the soil. However, when toxic chemicals are seeping ever closer to municipal water supplies with each passing hour, time is of the essence.
The problem of uneven heat distribution in the soil has been addressed by inserting many more pairs of electrodes into the soil. This results in many more sub-surface current paths along which soil can be heated. However, this solution results in the need to bore many more holes in the soil. The mechanical disturbances associated with boring these holes can affect the sub-surface properties of the soil in a way that accelerates the dispersal of toxic chemicals.
What is therefore desirable in the art is a method and system for evenly heating contaminated soil while minimizing the number of electrodes inserted into the soil.
A soil remediation system according to the invention generates a sub-surface rotating field that drives currents within a remediation zone. Because the magnitudes of these currents are responsive to the sub-surface field distribution, and because the sub-surface field is a rotating field, the current density within the remediation zone, when integrated over time, is relatively uniform. As a result, the soil remediation system of the invention uniformly heats the contaminated soil.
A soil remediation system for heating contaminated soil includes three electrodes in electrical communication with the contaminated soil and positioned approximately 120 degrees apart on the circumference of a remediation circle. Each electrode is driven by an AC voltage. The second and third electrodes differ in phase from the first electrode by 120 and 240 degrees respectively.
To enhance its safety, the soil remediation system can further include a neutral electrode in electrical communication with the first, second, and third electrodes. This neutral electrode is typically disposed at the center of the remediation circle.
The electrodes, including the neutral electrode if one is present, can be positioned to ensure an overlap between the remediation zone generated by the electrodes and the region of contamination. Where the contaminated region is parallel to the earth""s surface, the first and second electrodes penetrate the earth to the same depth. However, where the contaminated region is not parallel to the earth""s surface, the first and second electrodes can be inserted at different depths. This tilt in the angle of the remediation circle relative to the earth""s surface changes the geometry of the remediation zone to match that of the contaminated region.
The electrodes themselves can be partially sheathed by an insulating jacket so as to form an insulated section and an uninsulated section in electrical communication with the insulated section. Preferably, the insulating jacket can be moved along a longitudinal axis of the electrode so as to adjust the relative surface areas of the insulated and uninsulated sections, thereby permitting further adjustment of the geometry of the remediation zone.
In another aspect of the invention, a soil remediation system includes a plurality of voltage sources that are operable with phase differences relative to each other. Equivalently, the soil remediation system can include a single voltage source with a plurality of phase delayed outputs. The voltage sources are connected to a corresponding plurality of electrodes that are disposed at selected locations in the contaminated volume. The phase delays between sources and the locations of each electrode are selected so to generate a sub-surface rotating field.
The invention also includes a method for heating contaminated soil by generating a rotating electromagnetic field within the soil. This is achieved by inserting first, second, and third electrodes along the circumference of a remediation circle and applying first, second, and third voltages to the first, second, and third electrodes respectively, thereby generating a sub-surface rotating electromagnetic field.
Preferably, the first, second, and third electrodes are disposed 120 degrees apart along the remediation circle and the electrodes are excited by voltages that are likewise 120 degrees apart. This can be achieved by applying a phase difference of 120 degrees between the first voltage and the second voltage, and applying a phase difference of 240 degrees between the first voltage and the third voltage.
Because not all contamination zones have the same shape, the method also includes the positioning of the first, second, and third electrodes to generate a remediation zone having a specified geometry. This can be achieved by selecting first, second, and third penetration depths for the first, second, and third electrodes respectively. Further refinement of the shape of the remediation zone can also be achieved by providing the first electrode with an insulated section having an insulated surface area and an uninsulated section having an uninsulated surface area, the uninsulated section being in electrical communication with the insulated section. The insulated and uninsulated surface areas can then be adjusted to form a remediation zone having a specified geometry.